A sound processing device such as a microphone array including a sound input unit using a microphone such as a condenser microphone and performing various sound processes based on the sound input to the sound input unit has been developed as a device to be incorporated into a system such as a mobile phone, a car navigation system or a conference system. Such a sound processing device performs a sound process such as a process of, for example, performing level control for sound signals generated based on the sound input to the sound input unit in accordance with the distance between the sound processing device and a sound source. By the level control in accordance with the distance from the sound source, the sound processing device may perform various processes such as a process of approximately suppressing a distant noise while maintaining the level of a voice produced by a speaker near the sound input unit and a process of approximately suppressing a neighborhood noise while maintaining the level of a voice produced by a speaker in the distance.
The level control in accordance with the distance from the sound source is performed by utilizing such a characteristic of the sound that the sound from the sound source propagates in the air as a spherical wave while it approaches a plane wave as the propagation distance becomes longer. Accordingly, the level (amplitude) of a sound signal based on an input sound is attenuated inversely proportional to the distance from the sound source. Hence, the longer the distance from the sound source is, the smaller the attenuation rate of a level with respect to a certain distance becomes. Assume that, for example, the first sound input unit and the second sound input unit are arranged with an appropriate interval D along the direction of the sound source, and the distance from the sound source to the first sound input unit is indicated as L while the distance from the sound source to the second sound input unit is indicated as L+D. The difference (ratio) of the levels between the sound input to the first sound input unit and the sound input to the second sound input unit is indicated as {1/(L+D)}/(1/L), i.e., L/(L+D). Here, it is estimated that the level difference L/(L+D) increases as the distance L becomes longer, since the distance L with respect to the interval D increases as the distance L from the sound source becomes longer. In the sound processing device, such a characteristic is utilized to approximately realize the level control in accordance with the distance from the sound source by converting each sound signal generated at each of the plurality of sound input units into a component on a frequency axis, obtaining the difference in levels of the sound signals for each frequency, and amplifying/suppressing a sound signal for each frequency in accordance with a distance based on a level difference.
According to the Japanese Laid-open Patent Publication No. 11-153660, a technique related to an acoustic process based on sound processing device including a plurality of sound input units is proposed.
When a process is performed based on the sounds input to a plurality of sound input units, it is desired for a plurality of microphones used as sound input units to have the same sensitivity. In generally-manufactured microphones, however, a sensitivity difference of, for example, approximately ±3 dB is generated even for nondirectional microphones having a comparatively small difference in sensitivity among them, presenting a problem that it may be preferable to correct the sensitivity in use. This causes a problem of increase in manufacturing cost if the sensitivity is corrected by manpower before microphones are mounted on the sound processing device. Moreover, microphones are deteriorated with age, and the degree of the aging deterioration varies for each microphone. Even if the sensitivity is corrected before being mounted, the problem of the sensitivity difference by aging deterioration will not be solved.